Increased awareness of the detrimental health consequences of NOx as well as tighter legislation have required improved adsorption and trapping systems for NOx.1 This invention concerns improvements relating to NOx traps or materials for the specific adsorption of nitric oxide (NO) and nitrogen dioxide (NO2) components in a gas mixture. Such systems are typically used in harm reduction for situations involving combustion processes, such as those that result in tobacco smoke. While gases arising from combustion processes can contain many components such as aldehydes, cyanides, sulphides and oxide, it is difficult to remove NOx, and especially NO, due to intrinsically low reactivity. A desirable characteristic of a NOx adsorbent is that it removes virtually all NOx present in a gas mixture with rapid kinetics. An additional desirable trait is that it functions at low temperatures—including room temperature and below. A furthermore desirable trait of such an adsorbent material is that it be tolerant to other molecules, particularly sulfur and sulfides, which are both known to act as poisons for metal-based NOx adsorption and catalysis sites.
In the field of tobacco-smoke filter technology, one of the major technological problems is the reduction of NO, which has been implicated to have a role in lung damage and a variety of diseases in smokers, including chronic obstructive pulmonary disease and emphysema.1,5 A desirable trait of a tobacco smoke filter is an active site (e.g., the oNO oxidation site) being essentially metal free or wholly organic.
A different strategy for trapping NOx molecules that has shown recent promise is the use of hybrid organic-inorganic materials. This proposal addresses the development of novel materials capable of detecting or trapping NO and NO2 via selective adsorption on specific organic binding sites. These sites consist of immobilized molecular receptors on silica platforms, which interact with NO and NO2 at low concentrations in gas and liquid phases.
Recent attention has focused on using organic functional groups, and specifically organic radicals, as active sites for NOx adsorption. NO2 is known to react with nitroxyl radical sites to synthesize an oxoammonium cation via reactions shown below.2,3,4 Two NO2 molecules are removed from the gas phase per nitroxyl radical site: one due to nitrite salt formation, and the other for nitrite oxidation to nitrate, which consumes NO2 as oxidant and forms NO in the gas phase as a by-product. The resulting oxoammonium nitrate salt is known to be non-hygroscopic, and thermally, mechanically, and oxidatively stable.3 
Nitroxyl radicals have been immobilized onto polymers and high surface area porous materials in the prior art in order to remove NOx from a gas mixture; however, only a fraction of NO can be removed from a gas mixture with these materials, typically less than 72%.5

Nitronyl nitroxides are known to act as an oxidant in reacting with NO to form NO2.6 Materials consisting of physisorbed, non-covalently immobilized nitronyl nitroxides on the surface of silica have been used to previously convert NO to NO2.7 A significant limitation when using such materials is the very slow kinetics of NO oxidation, indeed too slow to be useful in a practical application, as it typically occurs over time scales of several hours at low NO concentration.7 It would be a distinct advantage to synthesize an immobilized nitronyl nitroxide-containing site that could react with NO almost instantaneously and at the limit of mass transport in bulk or mesoporous channels—typically involving fractions of a second for typical particle sizes.

A material that incorporated organic sites for adsorption or oxidation of NO, and/or organic sites for adsorption or reduction of NO2 (e.g., essentially simultaneous oxidation of NO and storage of NO2) would represent a significant advance in the art. Quite surprisingly, the present invention provides such materials.